1. Field of the Invention
This invention relates to a real image mode variable magnification finder which is suitable for use in a camera in which a finder lens is constructed to be independent of a photographic lens.
2. Description of Related Art
Finders used in cameras in which a finder optical system is constructed to be independent of a photographing optical system are roughly divided into two types: a virtual image mode finder and a real image mode finder. The virtual image mode finder has the disadvantage that the diameter of a front lens must be enlarged as a variable magnification ratio is increased, which constitutes an obstacle to compactness of the camera. Furthermore, the finder optical system of this type has the problem that the view of a field frame is impaired.
In contrast to this, the real image mode finder is such that its entrance pupil can be located on the front side of the finder optical system and thus the diameter of the front lens can be diminished. Moreover, since an image formed by an objective system is observed through an eyepiece system, the defects of the virtual image mode finder are practically obviated, and a finder which has a good view of the field frame can be realized. The real image mode finder, however, has the disadvantage that dust particles deposited in the proximity of the image plane of the objective system are superimposed on the visual field and observed. Between the objective system and the eyepiece system, image erecting members for obtaining an erect image are arranged, and dust particles adhering to a surface close to the image plane, of the surfaces of these members, will also be seen in sharp focus, together with the visual field.
In keeping with compactness and high variable magnifications of cameras in recent years, finders must also follow these tendencies, and real image mode finder optical systems suitable for such compactness and high variable magnifications are often used. Many of the real image mode finders which have relatively high variable magnifications such that a variable magnification ratio is more than 2 are designed so that the objective system is of a three-lens unit zoom type. In particular, a real image mode finder in which the lens units of the objective system are arranged to have negative, positive, and positive powers in this order from the object side has an objective system with a long back focal distance. This is very advantageous for placement of a part of the image erecting members in the objective system and for compactness and simplification of the arrangement of the entire finder system. The finders in which the lens units of the objective system are arranged to have negative, positive, and positive powers as mentioned above are set forth, for example, in U.S. Pat. Nos. 5,257,129, 5,552,931, and 5,757,543.
U.S. Pat. No. 5,257,129 and the second to fourth embodiments of U.S. Pat. No. 5,757,543 are such that prism surfaces are placed immediately before and behind a field frame (field stop) which is an image plane, and dust particles, when adhering to these surfaces, will also be observed in sharp focus. The first embodiment of U.S. Pat. No. 5,757,543 is constructed so that mirrors are arranged on the object side of the field frame and thus dust particles are liable to penetrate into the field frame from the side of an objective system which is movable. U.S. Pat. No. 5,552,931 is such that prisms are switched on the object side of the field frame and thus dust particles are liable to penetrate into the field frame.